"It seemed amusing for a while," he says, "but then people started to think of what could have been."

The authorities are reluctant to speculate about events that have not occurred, and the UK's unwritten constitution gives very little guidance. But if something catastrophic did happen in the Commons chamber, there are a few pointers as to what would happen.

It seemed amusing for a while, but then people started to think of what could have been

Andy OppenheimerWeapons expert

Any unaffected cabinet members would meet and hold a vote to determine who would act as the prime minister, spokespeople for both the Labour and Conservative parties say.

Then, the party would choose a new leader, according to their own processes. It's likely, however, that a new leader would come from the upper ranks of the Cabinet, if one was still around.

The Labour Party says the chances of the whole Cabinet falling prey to an attack simultaneously are too low to consider seriously.

Prime Minister Blunkett?

A fair point. During the purple flour incident, for example, neither Home Secretary David Blunkett nor Defence Secretary Geoff Hoon were at Westminster.

Deputy Prime Minister John Prescott and Chancellor Gordon Brown, who sat on either side of Mr Blair, were dusted by the flour as it wafted across the house.

The Labour party said the replacement process in such a scenario could be a "rather immediate" one.

Is it so bizarre to wonder that if things had been radically different, Mr Hoon or Mr Blunkett might have found themselves leading the country by this weekend?

Mr Blunkett was at a conference, and was not in the House

If this had happened in the United States, where there is a clearly defined order of presidential succession, this discussion would be far more straightforward.

There, an orderly queue is formed behind the president and the vice-president, and everyone knows their place.

Besides the political nightmare that would ensue after such an attack, there's the impact of the incident itself to consider.

Biological weapons

If it had involved a dangerous biological or chemical weapon such as anthrax, ricin, or a myriad of accessible chemicals, the public would undoubtedly be in a state of panic.

"You'd be talking about the biggest headlines since 9/11. Even if people didn't die as a result , the very idea of it happening at the heart of government just beggars belief," Mr Oppenheimer says.

"And such an attack is theoretically possible."

Had it been a dangerous substance, MPs would have been contained in the area, thoroughly decontaminated and given antibiotics in the case of anthrax.

The office of the sergeant at arms, which is responsible for security in the Commons, refused to say whose decision it would be to seal MPs inside the House in the event of a chemical or biological attack.

Police, fire, and ambulance arrived quickly on the scene

"We don't comment on security details," a spokesperson said.

But many experts have expressed astonishment that in this incident the room was cleared and MPs left wandering freely, apparently before the substance was tested.

"It was quite startling that they appeared to have had no training or briefing," said Pauline Marren of the Emergency Planning Society.

"Let's hope this wasn't a miniature picture of what would happen in the wider population."

Evil intent

Perhaps a more frightening scenario is where we would be today had someone with "evil intent" attacked the Commons covertly, said Mr Oppenheimer.

"A bigger problem, for example with anthrax, is when you don't know you have been attacked.

"They would be walking around depositing particles in the street and around other buildings."

With the worst type of anthrax infection, in which the spores are inhaled, it would be too late to save most victims by the time it was detected.

"The contamination and clean-up would be a major problem. It might sound strange to say but the problem could become less about how many people it killed, and more about the utter disruption it could cause."

It's now clear either that the review has failed to identify what appears to have been a fairly obvious security flaw, or that the flaw was identified but ignored

Charles Shoebridge

Anthrax spores being spread around the country isn't the only problem we could be dealing with today.

Had someone with a similarly good aim decided to assassinate Tony Blair or any other individual, a dart containing the poison ricin would have killed them, added Mr Oppenheimer.

"Ricin in an aerosol form could have killed half the Cabinet, although it would probably need to be in a more enclosed space."

Training

Charles Shoebridge, a security analyst and former counter terrorism intelligence officer, says: "I'm not sure whether it was depressing or reassuring to see from their reactions that politicians have had the same amount of terrorist incident training as the rest of the general public - virtually none."

"In March - two and a half years after the 9/11 attacks - Peter Hain, the Leader of the Commons, announced that a security review was underway. Two months later, it's now clear either that the review has failed to identify what appears to have been a fairly obvious security flaw, or that the flaw was identified but ignored."

One problem the review may face, says Mr Oppenheimer, is that no one really knows how biological and chemical weapons can be most effectively administered to cause multiple deaths.

"So much is educated guesswork," he says. "For example we know anthrax is lethal, but no one really knows how many people you can get with it.